A fuel cell as a power generation device that converts chemical energy of fuel into electrical energy by electrochemically reacting fuel gas and oxidant gas may be widely used as industrial, household, and vehicular power supplies and used even for supplying power of small-sized electric/electronic products and portable devices.
Currently, as the vehicular fuel cell, a proton exchange membrane fuel cell or polymer electrolyte membrane fuel cell (PEMFC) having high power density is mot researched.
The PEMFC adopts hydrogen as the fuel gas and oxygen or air containing the oxygen as the oxidant gas.
The fuel cell includes a plurality of cells that generates the electrical energy by reacting the fuel gas and the oxidant gas and is generally used in a stack type in which the cells are stacked and assembled in series in order to satisfy a required output level.
Even in the case of the fuel cell mounted on a vehicle, as a high output is required, hundreds of cells individually generating the electrical energy are stacked in the stack type to satisfy such a requirement.
Herein, when a unit cell configuration of the PEMFC is described, the PEMFC is configured to include a membrane electrode assembly (MEA) in which catalyst electrode layers are attached to both sides of a membrane based on a polymer electrolyte membrane in which hydrogen ions move, a gas diffusion layer (GDL) supplying the fuel gas and the oxidant gas as reaction gas to the MEA and transferring the generated electrical energy, a gasket and a joining mechanism for maintaining airtightness and appropriate joining pressure of the reaction gas and cooling water, and a separation plate (bipolar plate (BP)) moving the reaction gas and the cooling water.
Herein, the MEA is configured to include the polymer electrolyte membrane capable of moving the hydrogen ions, and a cathode and an anode as electrode layers to which the catalyst is applied, which allows the hydrogen as the fuel gas and the air (alternatively, oxygen) as the oxidant gas to react to each other on both planes of the electrolyte membrane.
The gas diffusion layers for evenly distributing the fuel gas and the oxidant gas are stacked in an external part of the MEA, that is, the external parts of the cathode and the anode and the BP is positioned in the external part of the gas diffusion layer, which provides a flow path through which the reaction gas and the cooling water pass and supplies the reaction gas to the gas diffusion layer.
A gasket for sealing fluids, and the like are stacked to be interposed between components constituting the unit cell and the gasket may be provided while being integratedly molded to the MEA or BP.
A plurality of cells are stacked by using such a configuration as the unit cell, and thereafter, end plates for supporting the cells are coupled to an outermost part and the end plates and the cells are together joined by using a stack joining mechanism while the cells are stacked and arrayed between the end plates to configure the fuel cell stack.
The fuel cell system mounted on the fuel cell vehicle includes the fuel cell stack and apparatuses for supplying the reaction gas to the fuel cell stack.
That is, the fuel cell system includes the fuel cell stack generating the electrical energy from the electrochemical reaction of the reaction gas, a hydrogen supply apparatus supplying the hydrogen as the fuel gas to the fuel cell stack, an air supply apparatus supplying the air containing the oxygen as the oxidant gas to the fuel cell stack, a heat and water management system controlling a driving temperature of the fuel cell stack and performing heat and water management functions, and a fuel cell system controller controlling an overall operation of the fuel cell system.
In the general fuel cell system, the hydrogen supply apparatus may include a hydrogen storing unit (hydrogen tank), a regulator, a hydrogen pressure control valve, a hydrogen recirculation apparatus, and the like, the air supply apparatus may include an air blower or an air compressor, a humidifier, and the like, and the heat and water management system may include a water trap, an electric water pump (cooling water pump) and a water tank, a radiator, and the like.
In such a configuration, high-pressure hydrogen supplied from the hydrogen tank of the hydrogen supply apparatus is depressurized at a predetermined pressure in the regulator and thereafter, supplied to the fuel cell stack and in this case, the depressurized hydrogen is pressure-controlled according to a driving condition of the fuel cell stack to be supplied to the fuel cell stack while a supply amount is controlled.
In the fuel cell stack, hydrogen which does not react but remains is discharged through an outlet of an anode (hydrogen electrode) of the stack or recirculated to an inlet of the anode of the stack by the hydrogen recirculation apparatus.
The hydrogen recirculation apparatus is an apparatus capable of increasing reliability of hydrogen supply and improving a life-span of the fuel cell and various recirculation methods are provided, but a method using an ejector, a method using a blower, a method using both the ejector and the blower, and the like are known.
The hydrogen recirculation apparatus recirculates unreacted hydrogen which cannot be used and remains in the anode of the fuel cell stack to the anode (hydrogen electrode) of the stack through a recirculation pipe again to promote reuse of the hydrogen.
In the fuel cell, as foreign materials including nitrogen, water, and steam which flow to the anode through the electrolyte membrane in the stack increase, the hydrogen amount in the anode decreases and reaction efficiency thus deteriorates, and as a result, hydrogen purge may be performed by opening a hydrogen purge valve installed on an exhaust line of the stack anode.
That is, the hydrogen purge valve for the hydrogen purge is installed on a pipe at the outlet of the anode of the fuel cell stack to allow the foreign materials including the nitrogen, the water, and the like to be discharged and removed together with the hydrogen in the anode of the stack, and as a result, a hydrogen concentration in the anode is controlled and maintained at an appropriate level to increase a hydrogen utilization rate.
When the foreign materials in the fuel cell stack are discharged, there are advantageous such as control of the appropriate hydrogen concentration in the anode, increase in hydrogen utilization rate, and improvement of a gas diffusion degree and reactivity.
Meanwhile, the durability life-span of the fuel cell stack is a very important element in securing merchantability of the fuel cell vehicle.
Therefore, various efforts for increasing the durability life-span of the fuel cell stack (preventing the deterioration of the stack are competitively made and various causes for the deterioration of the stack are also investigated.
A cause which forms a great part among them is loss of platinum (Pt) used as the catalyst that activates the reaction of the oxygen and the hydrogen in the stack.
When the loss of the platinum is continued and accelerated in the MEA, reactivity of the hydrogen and the oxygen deteriorates, and as a result, the performance of the stack deteriorates.
When the driving of the fuel cell is continued, the platinum is continuously oxidized and it is almost impossible to fundamentally prevent a phenomenon in which the oxidized platinum PtO or PtO2 is eluted, and as a result, the platinum disappears.
Therefore, an oxidization speed of the platinum is delayed or the oxidized platinum is rapidly reduced again before the oxidized platinum is eluted to delay a losing speed of the platinum.
It is important to maintain a potential to activate the corresponding reaction for delaying the oxidation speed of the platinum and reducing the platinum and a potential area in which the platinum may be oxidized and a potential area in which a platinum oxide may be reduced are distinguished to some degree based on the potential of the cells of fuel the cell stack.
In more detail, an oxidation reaction or a reduction reaction of the platinum occurs according to the potential of the cells of the fuel cell stack.
For example, in the cells of fuel cell stack, in an oxidation section (a voltage increase direction section) to a specific potential or higher, for example, 0.8 V or higher, the oxidation reaction of the platinum dominantly occurs and in a reduction section (e.g., 0 to 0.8 V, a voltage decrease direction) of 0.8 V or lower, the reduction reaction of the platinum dominantly occurs.
When a driving mode between 0.7 to 1.0 V is applied, the oxidation of the platinum occurs more frequently than the reduction of the platinum and when the driving is continued within such a potential range, the platinum losing occurs, in which as the platinum reacts with water (H2O), chemical dissolution occurs and the platinum is thus eluted.
It is experimentally and theoretically revealed that as the potential decreases, the reduction reaction is more actively progressed and when voltage distribution of actual driving vehicles is analyzed, the oxidation section in which average cell voltage is 0.8 V or higher mostly occupies 50% or more in a whole section, and as a result, it is very important to increase the frequency of the voltage decrease direction (reduction section) to increase a possibility that the platinum oxide will be reduced in order to reduce the loss of the platinum.
However, in the existing fuel cell hybrid system, the control degree of freedom for stack voltage is low and there is a high possibility that a range of the stack voltage which is primarily driven will be a voltage range in which the phenomenon in which the platinum is oxidized is dominant, and as a result, the existing fuel cell hybrid system is very disadvantageous in terms of the durability of the stack.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.